This invention relates to methods and apparatus to assist in the setting up of devices which produce output signals in quadrature, for example so as to ensure that the output signals are accurately in a 90.degree. phase relationship, or accurately of the same amplitude, or accurately with zero DC offsets.
Such a method and apparatus are useful in setting up a scale and readhead, such as used on co-ordinate measuring machines or machine tools, to determine the position of a movable part of such a machine relative to a fixed part. It is well known to provide such scales and readheads wherein the readhead has two outputs in quadrature. These outputs can be fed to further circuitry for counting the cycles of the incoming signals so as to determine position, for determining the direction of movement by determining which of the quadrature signals leads the other, and/or for providing position interpolation within one cycle of the output. Of course, it is desirable that the readhead should be set up correctly aligned relative to the scale such that the outputs are accurately in quadrature, with a 90.degree. phase shift, the same amplitudes and with zero DC offsets. Particularly in the case where the signals are to be fed to an interpolator, this can affect the accuracy of the resulting determination of the relative position.
An example of a scale and readhead providing quadrature outputs to an interpolator is shown in European Patent Application No. EP-0274841-A.
One way to determine such factors would be to feed the two quadrature signals to the X and Y inputs of an oscilloscope. This produces a Lissajous figure, which should be a perfect circle centred on zero if all of the above factors are correctly set up. The mechanical position of the readhead relative to the scale can be adjusted to achieve this. However, it is very difficult to judge whether the Lissajous figure observed is perfect. The results also depend upon the accuracy of calibration of the oscilloscope, for example ensuring that the gains of its X and Y channels are exactly matched and that neither introduces a DC offset.